In recent years, a light scanning device for making scanning with using an oscillating mirror which oscillates rotationally in reciprocating manner by a sinusoidally oscillating element such as a galvanometer has been proposed, instead of a light scanning device for making scanning by deflecting a light beam with a polygon mirror which rotates at a constant speed. In such light scanning device, the oscillating mirror is driven at a resonance frequency of the oscillating mirror in order to increase the oscillation amplitude. Although a size of the light scanning device can be reduced with using such oscillating mirror, the resonance frequency of the oscillating mirror changes due to temperature variations, so that the oscillation amplitude or phase of the oscillating mirror may be deviated from the reference value in some cases. Thus, a detection unit such as a BD sensor for detecting the light beam at least one point in the scanning range of the light beam is provided, and the irradiating start position and irradiating end position corresponding to the oscillation amplitude or phase at that time are calculated based on the detection timing of the light beam by the detection unit (e.g., refer to JP-A-2003-131151).